There is a filter for high-frequency electrical signals using magnetostatic waves that are spin waves. Such a magnetostatic wave filter includes a yttrium iron garnet thin film (hereinafter also referred to as a YIG thin film) epitaxially grown on a gadolinium gallium garnet substrate (hereinafter also referred to as a GGG substrate), and a dielectric substrate having an electrical ground electrode along the entire lower surface and two metal thin-film transducers on the upper surface. In the magnetostatic filter, the surface of the YIG thin film adjacent to the transducers on the dielectric substrate.
A high-frequency electrical signal, input to the Input transducer, generates a high-frequency magnetic field in the surrounding area (in vicinity). The high-frequency magnetic field generated in the vicinity of the input transducer induces magnetic moment precession in part of the YIG thin film. The induced precession propagates as spin waves to the output transducer through the YIG thin film. The spin waves having reached the output transducer induce a high-frequency electrical signal in the output transducer through the reverse process of the process at the time of the input. As a result, the magnetostatic filter functions as a band-limiting filter that passes electrical signals in the frequency band of spin waves that are excited by the input transducer and can be sensed by the output transducer.
Also, there has been a report about a magnetostatic filter using an output transducer formed with electrodes. Input electrodes are arranged at different distances from one another from the input transducer. The design, in which the lengths of the electrodes and the direction of current flowing in the electrodes are specified, gives the filter desired frequency characteristics.
However, the these filters have integration issues. The wavelengths of magnetostatic waves to be generated depend on the size of the electrodes of the transducers. For that reason, it is difficult to miniaturize those filters. Furthermore, spin-wave propagation media used in those magnetostatic filters are required to have such characteristics that spin precession should have long life-time. Therefore, a YIG thin film has been most often used. This material is a very serious hindrance to integration of the filters into LSI circuits (large-scale integrated circuits) by a hybrid circuit technique or the like. A YIG thin film is a crystalline thin film epitaxially grown on a GGG crystalline substrate. Epitaxial growth on a specific crystalline substrate cannot be realized by conventional LSI manufacturing processes, and integration of magnetostatic filters into LSI circuits is practically impossible at present.
Magnetostatic filters also have problems in characteristics. The frequencies of signals to be processed with a filter are the same as spin wave frequencies, according to the operating principles of transducers. Therefore, the propagation characteristics of magnetostatic waves in a YIG thin film are directly reflected in the characteristics of the filter. This puts great restrictions on the filter design. For example, to design a band-limiting filter, the cutoff frequency is limited by the magnetostatic wave excitation and detection ranges of the transducers on the high frequency side, and is limited by the frequency bandwidth of magnetostatic waves on the low frequency side. Particularly, the latter depends on materials. Therefore, it is difficult to widen the frequency band toward the low frequency side, as long as a YIG thin film or a material similar to a YIG film is used.